Commissioning of Photocathode Research Facilities at Daresbury Laboratory and First Results

1. Commissioning of Photocathode Research Facilities at Daresbury Laboratory and First Results Dr Tim Noakes Accelerator Physics Group, ASTeC Dr Reza Valizadeh Vacuum Science Group, ASTeC

2. Photocathode Research at STFC Daresbury Laboratory Photocathode research dates back to commissioning of the first photoinjector on the ALICE accelerator in 2006 ALICE (and EMMA) Photocathode Preparation Facility (PPF) 350 kV DC photoinjector using GaAs (Cs,O) photocathodes More concerted effort to improve performance including development of PPF in 2009

3. Metal Photocathode Research Interest in metal photocathodes stems from the establishment of VELA (and in the future CLARA) at DL VELA Accelerator Area 2.5 cell RF gun with polycrystalline Cu photocathode Experimental equipment for metal photocathode research being established!

4. Photocathode Research Equipment Three systems currently in use for photocathode research SAPI - Surface Analysis/Preparation Installation (more usually called the Multi-probe!) ESCALAB-II – Surface analysis facility TESS – Transverse Electron Energy Spread spectrometer

5. Multi-probe system for metal photocathode research Analyser X-ray Gun AFM LEED UHV Vacuum System X-ray Photoelectron Spectroscopy (XPS) - Compositional and chemical analysis Atomic Force Microscopy (AFM) - Surface roughness evaluation Low Energy Electron Diffraction (LEED) – Crystalline ordering (single crystal only) Kelvin Probe – Contact potential difference work function measurements Quantum Efficiency measurements – UV LED light source and calibrated photodiode Commissioning this instrument is first milestone of sub-task 12.5.3 (RF photocathodes) for EuCARD2 project!

6. Commissioning Data Wide Energy XPS scan of clean Ag AFM images of polycrystalline Cu Roughness and composition likely to be key parameters in determining photocathode performance

7. ESCALAB-II Instrument • UHV modified ESCALAB-II instrument • High resolution XPS • Low resolution Scanning Electron Microscope • Quantum efficiency measurements (UV LED and calibrated photodiode) • Kelvin Probe (temporary) Experiments on Cu and other metal photocathodes underway

8. Cleaning Cu for the VELA photocathode Three treatments: Ar ion bombardment, O plasma cleaning, HCl etching XPS Data O plasma treatment provides thin protective oxide (can be transported through air) Annealing in-situ produces clean surface Initial Q.E. estimates for VELA ~2x10-5 (many assumptions made!)

9. Alternative Metal Photocathode Materials Screen a selection of metals • Commonly used for photocathodes • Widely used in accelerators • Low work function Measure XPS, QE and  • As loaded • After Ar ion bombardment • After heating to 200OC Also re-measure QE and  after XPS (before heating) XPS data for polycrystalline Ag

10. Preliminary Data XPS data for ion bombarded samples • Not all data collected yet (problems with heating and Kelvin probe equipment • QE not always higher when  drops •  can change after XPS (with time!) • Adsorption of residual gasses? • Morphological changes?

11. Theoretical Modelling of Photocathode Performance ASTeC Photocathode Research Team, STFC Scientific Computing, Imperial College • Density Functional Theory modelling of the electronic band structure of Cu • Use Spicer three step model to calculate photocathode properties • Investigate effect of adsorbates • Extend to other metals/alloys

12. Further Work on Metal Photocathodes INFN puck design • O Plasma cleaning of alternative metals • Single crystal Cu (and other) photocathodes • Thin film and metal alloy photocathodes • Modification of the ESCALAB-II to accept ‘INFN style’ photocathode pucks • Transverse and longitudinal energy measurements from metal photocathodes

13. Transverse Energy Spread Spectrometer (TESS) Cryo-cooled photocathode mount Retarding field analyser Laser path High resolution camera Spectrometer to make transverse and longitudinal energy measurements • Cryo-cooled photocathode holder • Retarding field analyser • Laser light source

14. Commissioning Data 60 V 230 V First images obtained from GaAs(Cs,O) photocathodes prepared in PPF ~2% Q.E. • No voltage dependence – instrumental function very small • Shorter wavelength light - electrons promoted from deeper within the valence band - higher Q.E., larger energy spread Work presented at FEL’13 in New York and ERL ‘13 in Novosibirsk

15. Metal Photocathodes on TESS • Sample transfer system between Multi-probe and PPF/TESS Magazine Vacuum suitcase Attached to PPF Cut-through • UV light source (266 nm Crylas Laser system) Currently being commissioned on ESCALAB-II!

16. Summary • Suite of equipment being developed at Daresbury for the preparation and characterisation of metal photocathode materials • Multi-probe (SAPI) • ESCALAB-II • TESS • First results • Cleaning of Cu for use in the VELA RF gun • Screening of alternative metal photocathodes • Future plans • Further work on metals (plasma cleaning, single crystals, alloys) • Adapting the ESCALAB-II for use with INFN cathode pucks • Transverse and longitudinal energy spread measurements

17. Acknowledgements ASTeC Deepa Angal-Kalinin, Boris Militsyn, Lee Jones, Joe Herbert, Reza Valizadeh, Keith Middleman, Mark Surman Daresbury Technology Department Ryan Cash and Barry Fell Loughborough University Mike Cropper, Sonal Mistry STFC Scientific Computing Department Martin Lueders, Barry Searle Imperial College Nick Harrison, Bruno Camino